Remus had been separated out, during the war. Peter had whispered to the Order, had put the suspicions in their minds, had even had Sirius thinking that Remus was a spy. he'd betrayed Remus and damaged him, too, but Remus was willing to overlook it if it meant catching Peter before he did any of it.
"We can change it!" he insisted. "What happened, what he did, we can change it. We can make sure it never happens. We can be like we were. Don't treat me like I'm some sort of doormat who doesn't know my friends. I know him, and ht's not lying to me---"